How do you think these mini's are trained to be ridden? You can't just expect to put a "small child" on the horse and train them. It's just not logical. I don't know very many 7 or 8 year olds that train horses, I'm sure they are out there but not a lot.
This is directly copied from their website...
Q. Do people ride Miniature Horses?
A. Miniature Horses should not be ridden by anyone over 70 pounds; however, Miniatures are extremely adept at learning to drive. Although small children are of the appropriate weight and many Miniature Horses are gentle and willing mounts, as with any combination of horse and child, caution is advised.